The present invention relates to an improved method of producing .epsilon.-caprolactam from cyclohexanone oxime in the gaseous phase using (B) .beta.-zeolites.
It is known that .epsilon.-Caprolactam is of great economic significance as the starting point for the synthesis of Nylon 6. In the classic production method primarily used today, cyclohexanone oxime is reacted in the presence of fuming sulfuric acid by way of the well known Beckmann rearrangement to form .epsilon.-caprolactam. A disadvantage of this method is the use of ammonia which is necessary for the neutralization of the sulfuric acid and which is associated with an accumulation of ammonium sulfate of up to 4.5 t per t caprolactam. In order to avoid this undesired byproduct, efforts have been made since the middle of the sixties to carry out the reaction with the aid of heterogeneous catalysis.
It is known that .epsilon.-caprolactam can be produced from cyclohexanone oxime in the gaseous phase on solid catalysts, e.g., on zeolites having an MFI structure.
However, X and Y zeolites and mordenites in the H form or doped with rare earth metals or transition metals deactivate extremely rapidly. 5-Cyanopent-1-ene, cyclohexane and cyclohexanol are extensively formed in the presence of these catalysts as byproducts. Efforts have been made to avoid the known problems. Temperatures below 400.degree. C. atmospheric pressure were suggested as process parameters. Furthermore, non-polar solvents such as cyclohexane, benzene or toluene are supposed to be more advantageous than the use of more polar solvents. The partial blocking of the acidic centers of the HY zeolite with Na did not result in an improvement of the results (M. Burquet et al., An. Quim. Ser. A 81 (1985) 259; A. Aucejo et al., Appl. Catal. 22 (1986) 187 and A. Corma et al., Zeolites 11 (1991) 593). However, the authors determined that strongly acidic Br.o slashed.nstedt acid centers with pKa.ltoreq.1.5 are necessary for the Beckmann rearrangement and that the selectivity-reducing byproduct 5-cyanopent-1-ene is formed both on the acidic centers as well as on the Na.sup.+ ions. Furthermore, the authors surmise that the deactivation of the zeolite catalyst takes place as a result of basic byproducts such as aniline and methyl pyridine and not by the formation of coke on the catalytic surface since the color of the catalyst is still almost white after the reaction.
In order to avoid the disadvantages of the rapid ageing of the X- and Y zeolite catalysts, pentasil zeolites (MFI structure) were extensively investigated.
Zeolites with MFI structure are also regarded as favorable in the patents EP 0,494,535 and EP 0,544,530. However, a high Si/M ratio of 5 to 500 is required for these materials, wherein the metal can be Al, B, and Ga, among others. Alcohols and ethers as well as water are mentioned as added components. The maximum addition of water is approximately 0.06-2.5 moles per mole oxime.
The experiments for improving the service life of the catalyst also went in the direction that the outer surface of a boron-MFI zeolite was covered with Na ions in order to neutralize the acidity on the outer surface in this manner (EP 0,086,543 (May 17, 1986) Stamicarbon). A mixture of cyclohexanone oxime, toluene, carbon dioxide and water in a molar ratio of 1:3:7 at 340.degree. C. will react the cyclohexanone oxime quantitatively on this catalyst. The selectivity for .epsilon.-caprolactam is indicated as 58%.
It is also known that .beta. zeolites can be used in this reaction. However, the .epsilon.-caprolactam is produced only with a selectivity of 86.9% at a conversion of 80.6% (C.A. 127:278 152, JP-OS 97 241 237).
It is therefore an object of this invention to improve the method of producing .epsilon.-caprolactam by the Beckmann rearrangement in the gaseous phase with the aid of other catalysts.